The present invention relates to an information transmission apparatus such as a server used in a system for realizing service such as VOD (Video on Demand) in which stored media is reproduced at real time through a network, and a traffic control apparatus, and a method of managing bandwidth resources using the same and a method of admitting a call. Particularly, the present invention relates to an information transmission apparatus in a system in which information is transmitted by use of the storage media storing data, which is VBR (variable bit rate)-encoded, such as a DVD (digital versatile disk), a traffic control apparatus, a method of managing bandwidth resources using the same, and a method of admitting a call.
In recent years, an interactive video service for practical use is starting in accordance with high-speed and wide bandwidth resources of the network and the development of a digital image technique. VOD is one of typical example of such service. VOD can be explained as follows:                Specifically, one to one path connection is made between a video server having video sources and a receiver, which is called a set-top box. Thereby, an arbitrary video source is output based on a user's selection.        
VOD is excellent in its operability. However, in consideration of the method of using the path for corresponding to VOD, necessary processing ability of video server, and an amount of contents to be supported, the reduction of the cost, which is necessary for presenting service, becomes an important matter.
In the field of consumer electronics, the sale of DVD is near at hand. DVD is a new video disk standard in which video information such as a movie can be recorded onto an optical disk having the same size as a CD (compact disk) with high picture quality by MPEG2, that is, video encoding. Video players for DVD or its contents packages are expected to be sold at a reasonable price to be widely used. If DVD can be used as the above-mentioned video server, the buildup cost of the video server can be expected to be largely reduced.
The recording and reproducing method of DVD is based on the concept of variable bit rate encoding in view of the point that both the image quality and recording time to capacity are ensured. The amount of encoding bits depends on the image quality of the original picture. The faster the motion scene is, the more the amount of data increases. In contrast, the conventional VOD was usually based on a CBR (constant-bit-rate)-encoded video source. The main reason is that there is not yet established the method of transmitting variable-bit-rate encoded video data, that is, VBR video through a network. This is also true to the field of ATM in which the VBR transmission is already defined, although it is assumed that many of the VOL) service use an ATM (Asynchronous Transfer Mode) in at least a core part of the network.
In ATM, several service classes are defined. For example, the following five classes are standardized in an ATM forum, which is the industrial standard setting organization for ATM.
Specifically, there are a CBR (Constant Bit Rate), a RT-VBR (Real Tune Variable Bit Rate), a NRT-VBR (Non-Real Time Variable Bit Rate), an ABR (Available Bit Rate), and an UBR (Unspecified Bit Rate).
Since the video transmission service needs a real time property, the CBR, which can ensure a constant rate type traffic, is generally used in the present state. The study of using the RT-VBR has be recently started.
Among the above five classes, the CBR and RT-VBR are guaranteed in their quality in view of real time. The CBR has an extremely low probability of cell discard and a high quality. However, a statistical multiplexed effect, which is particularly useful as an effective using method of managing the bandwidth resource, is not used in the ATM, the transmission cost of the CBR is high. The probability of cell discard of RT-VBR is higher than that of the CBR. However, since the CBR uses the statistical multiplexed effect, the transmission cost is lower than the CBR.
In the ATM, a call admission is executed as follows:                Specifically, a terminal notifies a traffic parameter to a network, and the exchanger of the network determines whether or not a call is admitted. After the call admission, the network polices whether or not the terminal outputs the traffic as keeping the notified parameter, and discard a violated traffic.        
In the CBR, a notification parameter is a peak cell rate (PCR). The PCR is a parameter, which defines a maximum speed of one ATM connection. The bandwidth resource notified by PCR is constantly allocated to the network during the time when the call is set. In the CBR, since the bandwidth resource is sufficiently ensured, and the data communication is performed, a good image quality can be obtained. However, since no multiplex effect is used, the communication cost is high. In case when the VBR video such as DVD is transmitted, if the bandwidth resource is notified to adjust to the peak value of the variable rate transmission, the large number of portions where the allocated bandwidth resource is wasted is generated, the total transmission cost is unnecessarily increased. To efficiently use the allocated bandwidth resource, encoded data is once decoded, and re-encoded to set the source of the VBR to the source of the CBR. Or, the rate must be flattened by buffering. However, the encoding operation is generally executed in consideration of the buffer management of the receiver. Due to this, the above-mentioned flattening causes increase in the buffers of the receiver side, and the manufacturing cost is increased. In other words, there is actually difficulty in flattening the rate. In general, the rate is adjusted as described below to execute the re-encoding operation.
Specifically, data of the portion where bits are allocated at a rate, which is higher than a certain reference value, is partially deleted. Then, stuffing bits, which have no influence on decoding, are inserted to the portion where bits are allocated at a rate, which is lower than the certain reference value.
FIGS. 10A and 10B show the state of a rate adjustment process by the re-encoding operation. FIG. 10A shows a rate characteristic of the VBR-encoded video. As shown in FIG. 10B, data of the portion where bits are allocated at the higher rate than the PCR, which is the notification parameter of the CBR, is deleted (cross hatched portion of FIG. 10B). Then, stuffing bits are inserted to the portion where bits are allocated at the lower rate than the PCR (slanting lines of FIG. 10B). If the source of the VBR is transmitted at the CBR, the number of hardware for re-encoding is increased. Moreover, regarding the portion where bits are allocated at the higher rate in the original source, the amount of bits is decreased, so that the image quality is deteriorated.
The notification parameter of the RT-VBR, which is another service class, includes a sustainable cell rate (SCR) showing an average bit rate and a maximum burst size (MBS) corresponding to PCR durable time, in addition to the PCR. The exchanger uses an imaginary buffer (leaky bucket) and polices whether or not the buffer is full of information. In the leaky bucket, a maximum value of an input is restricted by the PCR, the output rate is the SCR, and the size is the MBS. Even if the VBR is used, data cannot always be transmitted in the form that the traffic characteristic of the VBR of the source is unchanged. The traffic characteristic must be shaped to the traffic characteristic of the VBR, which the network allows, when transmitting data. For example, suppose that a scene where bits are allocated at a rate close to the peak rate exists in the scenes recorded by DVD, and that the scene is contained for several seconds. In this case, time when the peak rate can continue at a transmission time is restricted by the MBS, and the transmission rate must be reduced for a certain period of time. This state is shown in FIGS. 11A and 11B. The cross hatched portion of FIG. 11B is data of a portion exceeding MBS. Regarding such data, the image quality is reduced due to the re-encoding operation. If the large MBS (maximum burst size) is allocated to prolong the period of time when the peak can continue, the wasteful allocation of the resource occurs similar to the case of CBR. This results in the rise of the transmission cost.
In the present notification parameter, in order to pass through the traffic, which the source originally has extra resources for safety must be provided to prevent congestion. This does not meet the demand for the manufacturing cost, and the traffic characteristic must be shaped in accordance with the network request. As a result, a hard load and the deterioration of the image quality occur.
Thus, in the framework of the existing network, when VBR data such as a motion picture is transmitted at real time in the existing ATM service class as a typical example, the following problems occur.
Specifically, in order to adjust the rate characteristic of encoded data to the CBR, encoded data must be once decoded and re-encoded in the CBR service class. As a result, the hard load on the server side occurs, and the image quality is largely deteriorated due to the re-encoding operation. Also, in the VBR service class, data cannot be transmitted at the rate characteristic, which the VBR source originally has, by policing. As a result, there occur the hard load and the deterioration of the image quality, even if which are not worse than the case of CBR. Therefore, the re-encoding operation and the rate adapting operation are needed.